(1) Field of the Invention
The present invention relates to heavy duty pneumatic tires with composite type tread patterns. More particularly, the invention relates to tread patterns which can reduce rib tears growing from cracks formed at bottoms of circumferential main grooves of tires at shoulder portions.
(2) Related Art Statement
Particularly, in actually manufactured heavy duty pneumatic bias tires, an angle of cords of a carcass ply forming a carcass line is limited to 60.degree. at the maximum, and is ordinarily set at 50.degree. to 60.degree. with respect to a tire meridian section. Furthermore, since a radius of a crown can not be made so small from the standpoint of maintaining wear resistance, a rubber gauge in a tread is distributed such that the shoulder portion becomes naturally thicker than the central portion. Thus, a greater amount of heat is generated at the shoulder portions during tire running, so that a separation problem occurs between the tread and the breaker.
In order to reduce a volume of rubber at the shoulder portions and restrain heat generation as countermeasures for solving the above separation problem, Japanese patent application Laid-open No. 60-255,507 (U.S. Pat. No. 4,567,929,) has already proposed a tire shown in FIGS. 3 and 4 in which lateral grooves extend axially outwardly or inwardly from edges of a tread.
As the volume of the lateral grooves arranged in the shoulder portions of the tire increases, effect of restraining the heat generation at the shoulder portions appears to a large extent. However, if the volume of the lateral grooves is increased without due consideration, the rigidity of shoulder land portions becomes smaller due to a positional relation to circumferential main grooves arranged in the shoulder portions. Cracks (E) illustrated in FIG. 4 occur in the circumferential main grooves, particularly, groove bottoms at their bent portions due to flexural deformation during tire rolling. Thus formed cracks grow in length and width during tire running. Thus, there occurs a rib tear problem that a shoulder land portion is cut off over a wide range.